Is your arrival date after the visa start date? That’s a big error I’ve had people have (entering before their visa because they were visa-free but then not being able to validate unless they lie or actually leave and re-enter properly). Otherwise it may be a website error. There’s usually a way to contact the relevant service (it’s been a while since I’ve had to validate a visa and never had this issue, so I can’t tell you exactly who or where or how, but clicking around on the website usually turns something up). This is a great example of « don’t put the thing off till later » particularly when it comes to France and anything that involves a website.